[Fine-structure changes in Toxoplasma gondii trophozoites after deep-freezing with dimethyl sulphoxide (author's...

The changes observed in trophozoites of Toxoplasma gondii after deep-freeze preservation were examined by electron microscopy. Toxoplasmas (strain BK) from peritoneal exudate of infected NMRI mice were supended in Ringer's solution, deep-frozen in liquid nitrogen with 5% dimethylsulphoxide (DMSO), and compared after thawing with control samples with and without the addition of DMSO. Slight structural changes such as widening of endoplasmic reticulum, formation of fissures in the cytoplasm, and loosening of chromatin were only observed in some of the free toxoplasmas of the DMSO control. Among the deep-frozen parasites, about 1/5 of the free stages showed no or only slight morphological changes. In contrast to this, almost all intracellular forms found in macrophages showed lesions. The most remarkable change was a partial destruction of the inner cell membrane complex. The outflow of ribosome-containing protoplasm with ballon-like swelling of the outer elementary membrane was observed as a consequence of this frequent lesion. The outflow of protoplasm induced a drastic decrease in the electronic density of the whole cytoplasm. Other characteristic degenerative signs were vacuolation of cytoplasm up to formation of great optically empty spaces, widening of the perinuclear space, swelling of mitochondria, disintegration of rhoptria, micronemata, and Golgi zone, coarse-plaque loosening, and displacement of electron-dense areas of the nucleus up to disintegration with maintenance of the karyoplasm. In some almost completely disintegrated trophozoites, enlarged mitochondria with remarkable electronic density were observed. Apart from the cell membrane, the conoid was the longest-persisting organelle. The alterations observed after deep-freezing permit the conclusion that the free cells, which were only slightly impaired or not at all, remained infective.